Vacuum tube system



July 17, 1934- R. D. BROWN, JR 1,966,607

VACUUM TUBE 'SYSTEM Filed MaIC 21. 1931 y 3 Sheets-Sheet l July 17,1934- R. D. BROWN, JR 1,966,607

VACUUM TUBE SYSTEM F11-ed March 2'1. 1931 s sheets-sheet 2 fsb@ Julyy17, 1934. R. D. BROWN, JR 16,966,607

VACUUM TUBE SYSTEM Filed March 21. 1931 3 Sheets-Sheet' 'V3 L 2' /Qg'-]0T/ JE `ll l Patented" July 17, 1934 UNED STATES PAT-ENT orrics 1 Claim.

This invention relatesto signaling systems utio corresponding to Zerogrid potential, regardless of grid potential amplitudes.

Another object is to provide a novel method of and means for limitingthe output current of a therrnionic Ydevice to la certain predeterminedvalue.

A still further object is to provide a new andA novell method of andmeans for 'controllinglthe operation of a vacuum tubeand itsassociatedcircuit.

Other objects of andvarious uses for my invention will be apparent fromthe following description.

In the drawings:

Fig. 1 shows a simple vacuum tube circuttol which my invention has beenapplied;

Fig. 2 illustrates graphically. the operation of Y the system of Fig. 1;

Fig. 3 shows a system for controlling the operation of the vacuum tubeby differentially vary.-v

ing the ratio of the grid biasto plate potential;

Fig. 4 illustrates graphically the `operation of the system of Fig. 3;

Figs. 5 to 7 show the application of my inven tion to various push-pullarrangements; o

Fig. v8 is a graphical illustration of the working of thetubes of Fig. 7in push-pull relation;

Fig. 9 shows a modificationof the system of Fig. 3; and o Fig. 10 showsa practical system embodying features of the other figures.

Referring to Fig. 1, there is shown a vacuum tube V having the usualsources of energization A, B and C. A source'of yoscillations Oisconnected in the grid-cathodecircuit ofthe tube. This source may beany source of high or low frequency impulses. The anode-cathodecircuitof the tube is represented -as having included therein a resistance RLwhich is intended to represent any load fed by the tube. A resistance R,

which mayfor example have a value of 5 megohms is connected between thesource O and the n grid of tube A further object is to provide a simplemethod.

(Cl. 25o-w27.)

Referring to Fig. 2 for the operation of the fundamentalvcircuit of Fig.l, there is shown the` characteristic curve of tube Vvobtainedvbyplotting plate current against grid voltage for a certain platepotentialB. Let us assume; that the grid of tube V is unbiased, that is,that the battery C has been removed, and that no oscillations are beingdelivered by vsource O. Then for all intents andpurposes the gridwillbeata low xed potential with respect to the cathode, and a steady65.*; plate current IsV willflow. Now if oscillations be ldelivered bysource O, the gridY will alternately be chargedpositively andnegatively. Upon becoming positively charged, the grid will drawcurrentand a potentialdropwill take place across the g, resistor R. Since thisdrop is directlypropor-A tional to the current which in turn dependsuponl the voltage of source O, it will practically neuf' tralize thepotential of sourceO. Forexample, if. the positive potential, applied tothe grid by A source O be assumed as 5 volts, then if R equals k5megohms and if internal resistance of the tubebe neglected, since it isso small `compared to R, the current .will equal 1 microampere. The.drop across Rv will then be 5 volts and the grid will be maintained atits original potential. In other words, as shown in Fig. 2, theplatecurrent will never rise appreciably above its steady value. Negativegrid charges will, -of course, cause' no grid .current flow, and henceno voltage dropI across R. As shown in Fig. 2, only .thenegative half ofeach incoming cycle will be repeated in the output circuit. l

The obvious result of the above discussed ac,.. tion is that theentireupper half of the char,` acteristic curve of ',tube uV has beenvlremoved and a substantially straight horizontal'line has beensubstituted therefor. In other., words, .a straightline `cut-01T hasbeen given the plate current. f However, the vupper portion of. Athe195curvemay be given any desired one of several4` various forms'byvaryingthe valuelof R. The broken lines fand in Fig. 2 indicate con-y ditionsAfor .considerably smaller vvalues of R. The gridomaybebiased in anydesired manner,- and four different .bias points ajb, c and, d have beenindicated in Fig. 2. For bias` c,r the tube would be effectivelyblockeduntil vthe amplitude increases to Aa value greater than c-b. For'biajsb, impulses of plate current wouldbe caused onlyby. positiveoscillations from theV source o. Operating at eitherh or c, the plate.4current would be limited to the value at o. Fork bias a', the upperportion of the plate current impulses caused .by positive `oscillationswould be limited due to the action of resistance R. Impulses due tonegative oscillations would be limited by the zero value or platecurrent. For bias d, impulses of plate current would not be caused bynegative oscillations until the value o-d is exceeded, positiveoscillations being totally ineffective due to resistance R.

Many practical applications of the system are possible. For example,operation of the tube V as a detector about the point o gives straightline detection at modulation for relatively small carrier waves. Anadded feature is that impulses of excessive value as for instance thoseexceeding o-b, will be limited by the zero value of plate current. Thetube may, of course, be used as an amplifier operating about point a',and is particularly adapted to amplifying code signals, since the topsof impulses of excessive Value may be chopped off if desired to give asubstantially square wave form while amplifying signals inthe rangeaf-o, a-b without undue distortion. Since the limiting action of thetube depends upon the value of resistance R, the value of bias source Cmay be adjusted to cause the tube to yblock oscillations of anypredetermined amplitude as described above. For instance, adjustment ofC to the values o-b will provide a detector with a definite maximumoutput limit. Many other applications are possible.

In Fig. 3, there is shown a system similar to that of Fig. 1 with anarrangement for varying the plate voltage and grid bias in adifferential manner. A battery B and potentiometer RB are provided toobtain varying supply voltages across the resistor R1 R2. By-passcondensers may be used across RB, R1 and R2 if desired. One side of thisresistor is connected to source O` while the other side is connected totransformer T which in turn is connected to load RL. The resistance ofR1 R2 should be relatively low in order that the plate current may below compared to the supply current in R1 R2. The cathode of the tube isconnected to a movable contact which may vary the ratio of R1 to R2. Itwill be apparent that the voltage across R1 is grid bias voltage whilethat across R2 is plate voltage. In Fig. 4 the family of curvesobtainable by varying the supply voltage of tube V is shown. Thesecurves may be represented as substantially straight lines between thetwo axes, although in practice the lower portion of each is slightlycurved. It will be apparent that the tube V can be made operative on anyof the characteristic curves by varying the setting of potentiometer RB,thereby varying the supply voltage. Varying the position of the cathodecontact will vary the ratio of the grid bias voltage to the platevoltage. For any particular setting of the cathode contact, the pointson the curves which represent the ratio R1:R2 for each value of supplyvoltage lie in a straight line, since the ratio R1:R2 remains constant.Three broken lines represent the loci for points representing differentvalues of the ratio R1:R2. The eiect then of changing the position ofthe cathode Contact, is to cause the locus of a point representing acertain ratio R1:R2 to revolve about the origin O between the axes Egand IP, as indicated by the arrows.

In the above explanation, RL was assumed to be coupled into the circuitin such manner that no substantial direct current potential dropoccurred across it. When RL is directly in the plate supply circuit,another condition occurs. With the load resistance RL set at a very lowvalue, potentiometer RB may be adjusted to give a certain steady platecurrent for' Zero grid' potential', such as e. This establishes point eon the curve, and if the adjustment of RB is maintained constant, thetube will work along curve d and the zero plate current point will beestablished at point f. Now if the load resistance RL is increased, thecurve will take the form indicated by dotted curve h, since theincreased resistance RL decreases the plate current but, of course, hasno effect at Zero plate current. Thus, thepoint f is rst established byadjusting potentiometer RB, then point i is established by varying; RL.In other words, it might be said that curve d is revolved about point fas a center of rotation by varying RL. This applies, of course, to` anyof the curves, although only two dotted curves h andy are shown in Fig.4. Because no substantial change of grid bias occurs when the value ofRL is varied, the three broken lines above referred to as representingloci of points for different values of R11R2 will take up new positionsso that they cut curves h and j at the same values of grid bias at whichthe threet corresponding lines cut d and o.

The above description of the device of Fig. 3 assumed, as abovementioned, that the supply current through R1R2 was large with respectto the plate current. However, the device will operate when the platecurrent constitutes the major part of the current through R1 in whichcase the efficiency of the device will decrease for the followingreason. An impulse from source O which adds to the eiect of thepotential across R1 in determining the instantaneous value of platecurrent will be opposed by the potential set up by the resulting changedplate current in R1. Consideration of this fact will show that in orderto obtain the same effect in the output circuit under this condition, ascompared with the condition where the supply current through R1R2 ishigh, an increased impulse from source O will be required. The extremecondition occurs when R2 is infinite. Thus, I have provided threeindependent adjusting means, one for the supply voltage, RB, which maybe considered a linear adjustment, one for the plate current, RL, andthe other for the ratio of grid bias voltage to plate voltage, whichlatter two may be considered rotary adjustments. The practical advantageof such a system is obvious.

In Figs. 5 to 7, I have illustrated the application of my invention tovarious push-pull arrangements. In Fig. 5 the tubes V1 and V2 areconnected with their input circuits in push-pull relation and theiroutput circuits feeding a common load represented by RL. The resistanceR is connected as shown so as to be common to` both grid circuits, andwill alternately prevent each grid from going positive. The source O maybe any source of high or low frequency oscillations. The action of thissystem is the same for each tube as described above in connection withFig. 1, the resistance R in this instance acting on each half of everywave to maintain one or the other grid at zero potential. The resultantoutput of the system is, of course, the sum of the plate currentsflowing through RL at any moment.

While two resistors, one in each grid leg, may be used instead of acommon resistor R, the use of a common resistor has a distinctadvantage. When the grid of tube V2, for example, tends to go positive,the resulting grid current flowing through the resistor R not onlymaintains the grid of tube V2 at zero potential, but also causes adoubled voltage to be applied to grid of tube V1. Since the polarity ofthe current flow through resistor R will be as indicated, it is apparentthat while `the voltage drop across resistor-R opposesv the potential.across the lower half of the input transferir. er secondary, it aids thepotential across the upper half of the secondary, thuscausing .avoltagedoubling action across the inputA of.

tube V1. vOn the next half: cycle the' grid of tube Vr will beinaintained at zero potential While theV voltage across V2 ivill'bedoubled. The advantage oi tbisvoltagedoubling action is obvious, since asignal amplitude of one-half the normal amplitude Will give the sameresultas a' signal of.' normal amplitude would give Without'thisaction.V

When used to detect signals, the systemof Fig. 5v

gives twice the efficiency of they ordinaryr single` f or low frequencyoscillations O and their output circuits in push-pull relation acrossoutput coil L. Two resistor-s R1 and-R2 are shoivn` in the respectivegrid circuits. The resultant output ofthe system is the di'ierentialoutput of the tubes. The system is capable'o various practicalapplications. sisters Ri and R2 and the-relative bias voltages oi thetubes, theroutput can be governed at will.

ln Fig. l my invention is shown applied to an ordinary push-pullarrangement, the input and output circuits of tubes V1 and V2 being inpushpull relation. The resistor R is shown as common to both grids, asin Fig.A 5, but here also two resistors may oe used. However, the samevoltage doubling action as described in connection with Fig. 5 isobtained here also by using a common resistor. The resultantcharacteristic curve of the tubes is for the greater portion thereof astraight line, and the system functions much more efficiently as anamplifier than the ordinary pushpull circuit. rhis is illustrated inFig. 8, which shows the characteristic grid voltage-plate Ycurrentcharacteristic curves-of the tubes V1 and V2 and the resultant curve b,the latter being illustrated by means of a broken line. Due to thevoltage doubling action, however, the resultant curve is modified asindicated by curve a, which, as will be noted, is a straight line. b

Fig. 9 shows a modiiication of the system of Fig. 3 utilizing a screengrid tube. lIn this system, the

plate potential is maintained constant, While the d varied in adiiierential manner.

control` grid bias and screen grid potential are The arrangement is suchthat the plate current does not disturb the voltage ratio 'of R1 R2. Aconstant sourceV oi plate potential B is connected between the v cathodeand plate in the output or load circuit. A source oi potential S andpotentiometer Rs lare provided to obtain varying supply voltages acrossthe resistor R1 One side of this resistor is conneet-er1 to .source O,While the other side `is conv teuA to the screen grid.

to a movable contact which may vary the ratio of R1 to R2. The voltageacross R1 is contol grid bias voltage, While that across R2 is thevoltage applied to the screen grid.V The curves of Fig. 4 are applicableto this system also. `However, in this case the tube is made to operateon Elimination By varying the relativevalues of re- The cathode isconcationsV of this system are possible, the system being applicable toother types of tubes. It is only necessary that the control element biasvoltage and the space-discharge current be variable With respect toeachother. For example, in the systemV oi Figy9 the'screen. grid -mightbe movable, in which: case the Aspace discharge. current could bev'ariedby movingthe screen With respect to the cathode.

One ieatureof this invention which is oi great advantage is that due tothe presence of resistor R, the vload on the input circuit cannot becomegreater thanthat of R itself, which may be made of as high a value asnecessary.. Obviously also, if R has a high value, small variations inyits value will have no noticeable effectsuch as noise on the operationoi the circuitA with which it is associated. It isimportant to note thatno plate or screen grid supply current should iiow'through resistor R.It is essential also that no capacity yhaving an impedance so low as tobe comparable to the impedance of at the Working frequency, existbetween the gridand the vexternal circuit.

The device of thejinvention provides a simple method of deriving a D. C.voltage which is a measurev of the modulation of a given carrier at anygiveninstant. A device such as that shown `in 1 may be operated as alinear detector to provide a constant unidirectional voltage which is ameasure of the uninodulated carrier. This voltage may be amplied iinecessary and may be supplied across the terminals of R1 and R2 of adevice similar to that of Fig. While the audio frequency component issupplied in place of source o to the same device. Such a system isdisclosed in Fig. 10, wherein O designates a source of modulated carrierwaves. IThis source is coupled by means of radio transformer to a tuneddetector stage which includes detector tube D and a resistor R connectedvin its grid lead. The plate circuit ofthe detector tube includes theusual source of anode potential supply B and also a large resistor R3. Afilter arrangement comprising condensers C1 and C2 and choke coil L1 maybe used to by-pass radio frequency currents. The detector isunbiasedand, therefore, plate current of al steady value corresponding to zerogrid potential normally flows when no incoming carrier is present.

The output circuit of the detector is coupled through audio frequencytransformer T2 to a device similar to that shown in Fig. 3. As in Fig.

'3, a vacuum tubeV has a resistor R. connected nected to a load resistorRL which may be shunted by a meter or other device M. A potentiometerresistance RP is connected across resistance R3 and source B. Oneterminal of RP and a poten-v tiometer movable contact are connected tothe input'electrodesof a vacuum tube ampliiier V3,

Whose. plate circuit includes resistances R1 and R2 and also the usualplate voltage v supply source B1.

@With no carrier present to excite the system,

a certain plate currenty 'ows in the output of the detector tubecorresponding to zero grid potential, as will be clear by reference toFig. 2. This current flows through resistance R3 and sets up a potentialthereacross which opposes that of source B. Since R3 is large, a steadynegative potential may be applied to the grid of amplifier Va by meansof the potentiometer which biases this tube negatively to x the steadyvalue of plate current flowing through resistances R1 and R2 to apredetermined value. The potentiometer is as adjusted and source B1 isof such value that substantially zero plate current ows in the outputor" tube V3. Assuming now that an unmodulated carrier is sent out fromsource O, it will be obvious by reference to the illustration of Fig. 2that only the negative half cycles of such carrier will aiect the platecurrent of the detector if the value of R is suitable. Such half cycleswill, however, cause a decrease in the average plate current flowingthrough resistor Rz and a corresponding lessening of the negative biason the grid ot tube V3. r"he plate current of this tube Will then beincreased to a predetermined value. In other Words, the potential whichis established across the terminals of R1 and R2 is proportional to theunmodulated carrier. If now a modulated carrier is sent out from sourceO, the potential established across resistance RL is a measure of themodulation of such carrier. By properly proportioning the values ofvoltage and resistance, the unidirectional current in RL may be madeproportional to the percentage modulation. A very important feature isthat due to the presence of R', the' rnain'inuni current' in RL may beadjusted so that it will never exceed that corresponding to 100%modulation. This operates as av protection to meter M or other apparatusconnected to Ri. against the eects of electrical disturbances. vThevoltage across RL could be used to provide a source of control voltage,which will vary with the envelope of the audio frequency and which. Willbe substantially free from the effects of disturbances for any suitablepurpose.

Obviously my invention is not limited to any particular type of vacuumtube or circuit. t is to be understood that the" systems illustrated anddescribed are for the purpose. of disclosure only, and that my inventionis not limitedl thereto. Many changes and adaptations of my inventionwill be apparent to those skilled in the art. Accordingly, my inventionis to be limited only by the scope of the appended claim'.

I claim:

In an electrical system, a pair of thermionic devices, each having agrid and a cathode and input and output circuits, a source of signaloscillations, said input circuits being connected in parallel relationwith respect to said source and said output circuits being connected inpushpull relation, means biasing said grids to different extents, and agrid resistor for each of said devices to prevent electron current owingfrom either of said cathodes to its corresponding grid regardless of theamplitude of any oscillations from said source.

REYNOLDS D. BROWN, JR.

